This invention relates to blind fasteners, and in particular to blind fasteners wherein bulbing of the fastener is caused during installation of the fastener into a workpiece.
Blind fasteners (i.e. fasteners which can be installed by access to one side only of the workpiece) are well known.
Commonly a blind fastener comprises a tubular body having an elongated shank, a preformed radially enlarged body head at a one end (the head end) of the body, a stem having a radially enlarged stem head at one end, wherein the stem extends through the core of the body such that the stem head is at a tail end of the body remote from the head end, and an end portion of the stem protrudes from the head end of the body. The fastener is inserted through aligned apertures in the workpiece comprising workpiece members to be secured together, such that the body head abuts the near face of the workpiece and the tail end portion of the body shank protrudes beyond the remote face (the blind face) of the workpiece. An installation tool is then used to apply an increasing pulling force to the protruding portion of the stem relative to the body, the reaction force being supported by the body head, so that the stem head deforms the tail end portion of the body shank radially outwards and axially towards the body head, to form a blind bulb which abuts the blind face of the workpiece. The workpiece members are thus clamped together between the body head and the blind bulb. Usually the stem is then broken off flush with, or slightly inside, the head of the body, at a breakneck groove preformed at the appropriate position along the stem. The breakneck breaking load is greater than the load needed to completely form the blind bulb.
Blind fasteners which provide a high level of static and dynamic joint strength need to develop a high retained compressive force on the workpiece, between the body head and blind bulb. Accordingly it is desirable to have a relatively large preformed body head, and also to have a relatively large contact area between the blind bulb and the blindside of the workpiece. The area of contact between the blind bulb and the blindside of the workpiece is known as the blindside footprint.
Fasteners are also known, such as those available under the trade marks STAVEX or Masterfix K-Lock, which develop multiple bulbs on installation into a workpiece, therefore allowing the fastener to cover a large grip range, i.e. to be capable of securing together workpieces having varying ‘grips’, i.e. varying total thicknesses of workpiece members.
However, during installation of a multiple bulbing fastener, each bulb will be formed at a particular point of the installation, i.e. when the fastener has been pulled relative to the sleeve by a particular amount. At certain thicknesses within the grip range of a multiple bulbing fastener, the cross-sectional area of the bulb in contact with the rear face of the fastened workpiece will be very low, and for certain ‘intermediate’ workpiece thicknesses, there will be a “stand-off”, whereby minimal or no footprint is generated on the blindside of the workpiece, because installation completes before the complete formation of a particular bulb. This results in a substantial reduction in fastening strength provided by the installed fastener.
The above situation is illustrated in FIG. 1, in which installation of the fastener 2, having a body 4 and stem 6 with a stem head 8, has completed at a particular stage wherein a bulb 10 has been formed but wherein no footprint is formed on the blindside of the workpiece 14.
An ideal fastener would produce a single bulb on the blindside of a workpiece, to prevent the above “stand-off” situation, and would do so consistently over a large grip range.
Certain known fasteners such as those available under the trade marks AVIBULB and AVINOX, employ a twin tapered fastener body form which forms a single bulb on the blindside of the workpiece, to achieve a wide grip range. This form of opposing tapers, as illustrated in FIG. 2, can be likened to a “barrel shape”. The tapered form is rolled onto the body 4′ of the fastener 2′ of FIG. 2 such that the minimum inner diameter of the body 4′ is approximately equal to the outer diameter of the fastener stem 6′. The body 4′ is then assembled onto the stem 6′ by a push-fit.
As can be seen in FIG. 2, a consequence of the above design is that there is a significant amount of clearance (indicated at 12) between most of the body 4′ and the stem 6′. Clearance between the body 4′ and the stem 6 is not desirable as it can lead to tilting of the stem 6 and the bulb which is formed during installation of the fastener into a workpiece.
A further disadvantage of such multiple bulbing fasteners as AVIBULB and AVINOX is the stem must be dimensioned such that the outer diameter is no greater than the minimum inner diameter of the rolled body to facilitate assembly of the body and stem. The pulling force for installing the fastener is therefore limited by the maximum allowable diameter of the stem 6′, and the shear strength offered by the installed fastener is accordingly limited.
Furthermore, a limited outer diameter of fastener stem also results in a risk of failure, i.e. breaking of the stem, at positions other than the breakneck during installation, a limited area of contact for the jaws of the installation tool which can lead to jaw slippage, and a limited resistance to bending forces during fastener assembly of the fastener and during installation into a workpiece.
It is an aim of the present invention to overcome or at least mitigate the above problems and provide an improved blind fastener which produces a single bulb, having a large blindside footprint, consistently over a wide grip range.
Accordingly, the present invention provides, in one aspect, a blind fastener according to claim 1 of the appended claims.
The bulb resulting from installation of the blind fastener of the present invention provides a single blindside bulb over a large grip range. In the present invention, the diameter of the stem is not restricted by the minimum inner dimension of the body before installation, and accordingly the pulling force required to install the fastener is not limited. As a stem having a greater diameter can be used, the potential for the stem breaking at a position other than the breakstem groove is reduced, the risk of installation tool jaw slippage is reduced, and the resistance to bending forces during assembly of the fastener and installation into a workpiece is increased. The present invention therefore provides a fastener which can be reliably installed to form a fastening having a high shear strength, over a large grip range.
After crimping of the body to cause material of the body shank to enter the first and second waisted regions, a barrel shape is formed by the body shank, thereby ensuring that on installation of the fastener into a workpiece, the bulb is formed in a controlled and predictable manner. The barrel shape formed by the body shank of the present invention is improved over that of prior art embodiments as the waisted regions allow the body shank to be deformed down to a reduced diameter but the remainder of the stem to remain at a diameter greater than the waisted regions.
A tapered portion is provided leading up to at least one of the waisted regions. The tapered portion is preferably located between the breakneck groove and the second waisted region, and has the effect of minimising the clearance between the internal surface of the body core and the stem outer surface. An advantage of reducing the clearance between the body and stem is that the potential for tilting of the stem or of the bulb formed during installation of the fastener into a workpiece is decreased.
The first waisted region may be located adjacent the stem head and the second waisted region located further away form the stem head than the second waisted region is located, such that the breakneck groove is between the first waisted region and the second waisted region.
A stepped portion may be provided between the first waisted region and the breakneck groove, the stepped portion having a diameter intermediate the minimum diameter of the first waisted portion and the diameter of the stem head. The stepped portion provides an increased resistance to removal of the stem from an installed fastener.
One or more radial projections may be provided on the stem at the first waisted region. The radial projections may have an asymmetrical triangular cross-sectional form and/or a rounded tip. The radial projections provide an increased resistance against removal of the installed fastener from a workpiece into which it has been installed.
The body of the fastener may include an annular underhead recess, providing easier clearance of burrs around the edge of the hole in the workpiece, and an enhanced fatigue strength of the installed fastener.
The first waisted region may comprise a plurality of stepped portions, thereby providing an increased resistance against removal of the installed fastener from a workpiece into which it has been installed.
The stem of the fastener may be provided with an annular underhead recess, whereby, on installation of the fastener into a workpiece, material of the body is forced into the recess, thereby preventing “stem head entry” failure wherein the tail end of the body spills out from the stem underhead region.
The stem may further comprise a protruding ridge of stem material adjacent the recess. The combination of the recess and the protruding ridge act to lock the body to the stem axially in the underhead region.
The present invention also provides, in a further aspect, a method of manufacturing a blind fastener. The method of manufacture includes forming a first and a second waisted region in the stem such that after the step of deforming the body, the body comprises a barrel shape, thereby minimising clearance between the body and the stem, and thereby reducing the potential for tilting of the stem and/or the bulb of an installed fastener.
The step of forming the first and second waisted region may comprise rolling, and the step of deforming the body may comprise crimping.
The method may further comprise the formation of one or more radial projections on the stem at a waisted region, wherein the step of deforming the body causes the at least one radial projection to embed into the body.
The first waisted region may be formed into a plurality of stepped portions.
The manufacture of the fastener may further comprise a step of forming an underhead recess. The recess may be formed in a cold-heading step, and the ridge may be formed by rolling, wherein material displaced from the first waisted region during rolling forms a protruding ridge of stem material adjacent the recess.